Electronvolt
In , the electron volt (symbol eV; also written electronvoltIUPAC Gold Book, p. 75[http://www.bipm.org/en/si/si_brochure/chapter4/table7.html SI brochure], Sec. 4.1 Table 7) is a unit of energy equal to approximately joule (Si unit J'). By definition, it is the amount of energy gained by the charge of a single moved across an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb, 1 J/C) multiplied by the electron charge (1 e, or ). Therefore, one electron volt is equal to .http://physics.nist.gov/constants Historically, the electron volt was devised as a standard unit of measure through its usefulness in sciences because a particle with charge ''q has an energy E=qV after passing through the potential V''; if ''q is quoted in integer units of the and the terminal bias in volts, one gets an energy in eV. The electron volt is not an SI unit and its value must be obtained experimentally.http://physics.nist.gov/cuu/Units/outside.html Like the on which it is based, it is not an independent quantity but is equal to (1 J/C)(2 / c0)0.5 It is a common unit of energy within physics, widely used in , , , and . It is commonly used with the SI prefixes milli-, kilo-, mega-, giga-, tera-, or peta- (meV, keV, MeV, GeV, TeV and PeV respectively). Thus meV stands for milli-electron volt. Atomic properties like the are often quoted in electron volts. In , it is often useful to have the , that is the energy that would be produced by one mole of charge ( ) passing through a potential difference of one volt. This is equal to . Energy Conversion factors: * = (the conversion factor is numerically equal to the expressed in coulombs). * (per atom) is . For comparison: * : Total energy released from a 20 kt Nuclear Fission Device. * ~624 EeV ( ): energy needed to power a single 100 watt light bulb for one second. (100 W = 100 J/s = ~ ). * 300 EeV ( ) = (50 J) :Open Questions in Physics. German Electron-Synchrotron. A Research Centre of the Helmholtz Association. Updated March 2006 by JCB. Original by John Baez. the so-called (the most energetic cosmic ray particle ever observed). * 14 TeV: the designed proton collision energy at the (which has operated at half of this energy ). * 1 TeV: A trillion electronvolts, or , about the kinetic energy of a flying .CERN - Glossary * 210 MeV: The average energy released in fission of one atom. * 200 MeV: The average energy released in of one atom . * 17.6 MeV: The average energy released in the of and to form ; this is per kilogram of product produced. * 1 MeV: Or, , about twice the -energy of an electron. * 13.6 eV: The energy required to ize . are on the of one eV per molecule. * 1.6 to 3.4 eV: the of visible light. * 1/40 eV: The at room temperature. A single molecule in the air has an average kinetic energy 3/80 eV. In some older documents, and in the name , the symbol BeV is used, which stands for billion electron volts; it is equivalent to the GeV. Momentum In , electron-volt is often used as a unit of . A potential difference of 1 volt causes an electron to gain a discrete amount of energy (i.e., 1 eV). This gives rise to usage of eV (and keV, MeV, GeV or TeV) as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of momentum units are ''M '''1 L 1''' T '''-1 . The dimensions of energy units are ''M '''1' L 2''' T '''-2 . Then, dividing the units of energy (such as eV) by a fundamental constant that has units of velocity (''M '''0' L 1''' T '''-1 ), facilitates the required conversion of using energy units to describe momentum. In the field of high-energy particle physics, the fundamental velocity unit is the speed of light ''c. Thus, dividing energy in eV by the speed of light in vacuum, one can describe the momentum of an electron in units of eV/''c''. The fundamental velocity constant c'' is often ''dropped from the units of momentum by way of defining units of length such that the value of c'' is unity. For example, if the momentum ''p of an electron is said to be 1 GeV, then the conversion to MKS can be achieved by: p = 1\; \text{GeV}/c = \frac{(1 \cdot 10^{9}) \cdot (1.60217646 \cdot 10^{-19} \; \text{C})\;\cdot\; \text{V}}{(2.99792458 \cdot 10^{8}\; \text{m}/\text{s})} = 5.344286\cdot 10^{-19}\; \text{kg}\cdot \text{m}/\text{s} Mass By , the electron volt is also a unit of mass. It is common in , where mass and energy are often interchanged, to express mass in units of eV/''c''2, where c'' is the speed of light in a vacuum (from [[Mass-energy equivalence|''E = mc2]]). It is often common to simply express mass in terms of "eV" as a unit of mass, effectively using a system of with c'' set to 1 (hence, ''E = m). For example, an electron and a , each with a mass of 0.511 MeV/c2, can to yield 1.022 MeV of energy. The has a mass of 0.938 GeV/c2. In general, the masses of all s are of the order of 1 GeV/c2, which makes the GeV (gigaelectronvolt) a very convenient unit of mass for : :: 1 GeV/c2 = 1.783 kg The atomic mass unit, 1 gram divided by , is almost the mass of a , which is mostly the mass of the proton. To convert to megaelectronvolts, use the formula: ::1 amu = 931.46 MeV/c2 = 0.93146 GeV/c2 ::1 MeV/c2 = 1.074 amu Distance In , a system of units in which the speed of light in a vacuum c'' and the ''ħ are dimensionless and equal to unity is widely used: . In these units, both distances and times are expressed in inverse energy units (while energy and mass are expressed in the same units, see ). In particular, particle s are often presented in units of inverse particle masses. Outside this system of units, the conversion factors between electronvolt, second, and nanometer are the following: : \hbar = = 1.054\ 571\ 726(47)\times 10^{-34}\ \mbox{J s} = 6.582\ 119\ 28(15)\times 10^{-16}\ \mbox{eV s}. The above relations also allow expressing the ''τ of an unstable particle (in seconds) in terms of its Γ'' (in eV) via ''Γ = ħ''/τ''. For example, the has a lifetime of 1.530(9) picoseconds, mean decay length is , or a decay width of . Conversely, the tiny meson mass mass differences responsible for are often expressed in the more convenient inverse picoseconds. Temperature In certain fields, such as , it is convenient to use the electronvolt as a unit of temperature. The conversion to kelvins (symbol: uppercase K) is defined by using k''B, the : : {1 \mbox{ eV} \over k_{\mathrm{B}}} = {1.602\,176\,53(14) \times 10^{-19} \mbox{ J} \over 1.380\,6505(24) \times 10^{-23} \mbox{ J/K}} = 11\,604.505(20) \mbox{ K}. For example, a typical plasma is 15 keV, or 170 megakelvins. Properties The energy ''E, frequency v'', and wavelength λ of a photon are related by : E=h\nu=\frac{hc}{\lambda}=\frac{(4.135 667 33\times 10^{-15}\,\mbox{eV}\,\mbox{s})(299\,792\,458\,\mbox{m/s})}{\lambda} where ''h is the , c is the speed of light. For quick calculations, this reduces to : E\mbox{(eV)}\approx\frac{1240\,\mbox{eV}\,\mbox{nm}}{\lambda\ \mbox{(nm)}} A photon with a wavelength of 532 nm (green light) would have an energy of approximately 2.33 eV. Similarly, 1 V would correspond to an infrared photon of wavelength 1240 nm, and so on. Scattering experiments In a low-energy nuclear scattering experiment, it is conventional to refer to the nuclear recoil energy in units of eVr, keVr, etc. This distinguishes the nuclear recoil energy from the "electron equivalent" recoil energy (eVee, keVee, etc.) measured by light. For example, the yield of a is measured in phe/keVee ( s per keV electron-equivalent energy). The relationship between eV, eVr, and eVee depends on the medium the scattering takes place in, and must be established empirically for each material. See also *Orders of magnitude (energy) References External links *BIPM's definition of the electronvolt *http://physics.nist.gov/cuu/Constants physical constants reference; CODATA data *Common Energy Calculator Category:Units of chemical measurement Category:Units of energy